PulsedLaserDepositionofBaTiO3 ThinFilmson …Pulsed laser deposition or PLD is a reliable method to...

Hindawi Publishing CorporationJournal of NanomaterialsVolume 2010, Article ID 756319, 5 pagesdoi:10.1155/2010/756319

Research Article

Pulsed Laser Deposition of BaTiO3 Thin Films onDifferent Substrates

Yaodong Yang, Zhiguang Wang, Jie-Fang Li, and D. Viehland

Department of Materials Science and Engineering, Virginia Tech, 306 Holden Hall, Blacksburg, VA 24061, USA

Correspondence should be addressed to Zhiguang Wang, [email protected]

Received 31 December 2009; Accepted 11 March 2010

Academic Editor: Christian Brosseau

Copyright © 2010 Yaodong Yang et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

We have studied the deposition of BaTiO3 (BTO) thin films on various substrates. Three representative substrates were selectedfrom different types of material systems: (i) SrTiO3 single crystals as a typical oxide, (ii) Si wafers as a semiconductor, and (iii)Ni foils as a magnetostrictive metal. We have compared the ferroelectric properties of BTO thin films obtained by pulsed laserdeposition on these diverse substrates.

1. Introduction

Pulsed laser deposition or PLD is a reliable method tofabricate oxide thin films [1, 2]. Deposition parameters playan important role in achieving good-quality thin films. KeyPLD deposition parameters include deposition temperature,laser pulse repetition rate, laser energy, and ambient gaspressure. Further, all of these conditions depend on thelattice parameters and atomic coordinations of the substrateand thin film material. Previous experiments have shownthat the substrate can be adjusted to change the qualityand nanostructure of the films to satisfy different needs[3]. For example, single-crystal substrates with differentlattice parameters can be used to tune the lattice mismatchof a film [4]: a suitable epitaxial strain can then enhancethe ferroelectric properties [4]. Clearly, understanding thebehavior of oxide films on various substrates is a meaningfulapproach to control the film quality and performance.

Barium titanate (BTO) is an important perovskite ferro-electric oxide due to its high-dielectric constant and largepiezoelectric coefficient. BTO thin films have been studiedfor many applications such as piezoelectric detectors, thinfilm capacitors, and magnetoelectric (ME) devices [3]. Inmost cases, SrTiO3 (STO) single-crystals are used as thesubstrate as it also has a perovskite structure and its latticeparameters are close to those of BTO [5]. It is easy to obtainepitaxial growth of BTO films on STO crystals. However, new

and promising applications have created a demand to depositBTO films on different types of substrates: these includesemiconducting Si and magnetic/magnetostrictive Ni.

Presently, silicon wafers are extremely utilized substrateto build electronic devices, such as complementary metal-oxide-semiconductor (CMOS) chips. If one could depositBTO thin films on the Si substrates, it would pave the wayto integrate BTO into microelectronic devices [6]. EpitaxialBiFeO3 films have previously been deposited on SrRuO3-(orSRO-) buffered Si by PLD [7]. Epitaxial BTO thin layer witha TiN bottom layer has also been deposited on Si by RF sput-tering technique [8]. These prior successes inspire us to finda method to grow BTO on Si by PLD technique. In additionto deposition, Si and metallic substrates offer other uniqueopportunities for deposition of BTO layers. In particular,growth of BTO on magnetic/magnetostrictive metals offersopportunity for multifunctionality [9]. Deposition of BTOfilms on metallic substrates and the study of the functionalproperties of each layer could be useful in the design ofnovel heterostructures, offering opportunities for BTO thinfilms in new applications such as magnetoelectricity ormagnetocapacitance. For example, the magnetostriction ofNi is ∼34 ppm at room temperature. If we could growa BTO film on Ni, one could then develop heterolayeredmagnetoelectric (ME) laminates: where open application ofan external magnetic field would induce elastic shapes in theNi foil, which would then be transferred to the BTO layer

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Typewritten Text

Copyright by the Hindawi Publishing Corporation. Yaodong Yang, Zhiguang Wang, Jie-Fang Li, and D. Viehland, "Pulsed Laser Deposition of BaTiO3 Thin Films on Different Substrates,” Journal of Nanomaterials, vol. 2010, Article ID 756319, 5 pages, 2010. doi:10.1155/2010/756319

2 Journal of Nanomaterials

generating a voltage. Such heterostructured ME laminateMetglas/PZT-fiber composites show giant ME voltages [10,11]. Deposition of BTO on Ni would offer a means tofabricate miniaturized engineered ME structures that doesnot require any epoxy bonding presently needed for thelaminated composites.

Here, after discussing BTO film deposition on (111)STO substrates, we will demonstrate successful growthof BTO films on (i) Pt-buffered Si and (ii) directly onmagnetostrictive Ni foil. We believe that our findings willhelp in the understanding of the role of various substrates onBTO films: ranging from insulating oxide to semiconductingSi and to magnetostrictive metals.

2. Experimental Method

We used (111)-oriented STO single-crystals, (100)-orientedSi wafers with a (111)-oriented Pt buffer layer, and magne-tostrictive Ni foils as substrates for the growth of BTO layersby PLD. The substrates were ultrasonically cleaned beforedeposition. A KrF laser of wavelength of 248 nm (LambdaPhysik 305i) was focused to a spot size of 10 mm2 and wasincident on the surface of a target using an energy density of1.2 J/cm2 for BTO. Deposition was done at an oxygen partialpressure of 100 mTorr using a laser pulse frequency of 10 Hzat a temperature of 750◦C. For the STO substrates, a 50 nm-thick SRO buffer layer was first deposited at 700◦C using anoxygen pressure of 90 mTorr which was used as a bottomelectrode for polarization measurements of the BTO film.Films grown on Pt-Si and Ni do not require an additionalSRO bottom electrode for these measurements.

Scanning electron microscopy (SEM) images wereobtained using a LEO (Zeiss) 1550 high-performance Schot-tky field-emission SEM. X-ray diffraction studies were per-formed using a Philips MPD high-resolution X-ray diffrac-tometer equipped with a two-bounce hybrid monochroma-tor and an open three-circle Eulerian cradle. FerroelectricP-E hysteresis loops were measured by a modified Sawyer-Tower circuit using triangle signals of frequency 100 KHz.Magnetization H-M hysteresis loops were measured by aVSM 7304 (Lake Shore Cryotronics) magnetometer.

3. Results

3.1. BTO on STO Single-Crystal Substrates. STO substratescan be used to grow epitaxial BTO piezoelectric layers wherethe film orientation is controlled by the substrate. First, as acontrol experiment to compare with layers grown on othersubstrates, we deposited epitaxial BTO thin films on (111)STO substrates. In Figure 1(a), a SEM cross-section imageof a BTO layer is shown: a uniform nanostructure can beseen, where BTO forms a columnar structure on STO withdiameters of about 80 nm. XRD revealed good epitaxy ofthe BTO layer on (111) STO, as identified by the line scanin Figure 1(b), where only a (111) BTO peak at 38.75◦ wasfound next to the (111) STO peak at 39.84◦. Clearly, wesuccessfully deposited epitaxial BTO thin films on top of(111) STO.

200 μmBTO

STO

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Figure 1: (a) SEM, (b) XRD, and (c) P-E loop of BTO thin filmgrown on STO substrate.

Figure 1(c) shows a polarization hysteresis loop of BTOon STO. A remnant polarization of Pr ≈ 23μC/cm2 wasfound which is very close to the single-crystal BTO value of

Journal of Nanomaterials 3

26 μC/cm2. The saturation polarization of Ps ≈ 60μC/cm2

was also comparable to that of BTO single-crystal; however,the coercive field of Ec ≈ 1.86 MV/m was much larger thanEc ≈ 0.1 MV/m for BTO crystals. The larger Ec value forthe film can be attributed to the epitaxial strain from thesubstrate and the resultant elongation of the BTO c-axis [12],requiring higher field levels for polarization reversal.

3.2. BTO on Si Substrates. Bare Si wafers are easy to oxidizeat high temperature, so we used Pt-buffered Si and studiedthe buffer layer effect on BTO thin film growth. Figure 2(a)shows a cross-section SEM image of BTO on Pt-buffered Si.BTO columns of about 100 nm in diameter can be seen. TheXRD line scan in Figure 2(b) shows good epitaxy of the BTO(111) peak at 38.8◦ on the Pt (111) at 39.9◦. Other peaks werenot present for BTO in the scan, indicating good epitaxialgrowth of single-crystal BTO layers. The Si (100) peak wasat 69.13◦. The reason for (111) BTO growing on (100) Siis that Pt forms a highly oriented (111) buffer layer, whichsubsequently has a small lattice mismatch with the (111)BTO lattice parameter.

A well-defined polarization hysteresis loop is shown inFigure 2(c) with Pr ≈ 7μC/cm2 and Ps ≈ 27μC/cm2, whichare much higher than the values previously reported forpolycrystal BTO on platinized Si substrates [13–15], againconfirming the high quality of epitaxially grown single-crystal BTO films on platinized silicon wafers.

3.3. BTO on Ni Foil Substrate. As an important and widelyused electrode metal in the electronics industry, Ni wasselected as a metal foil substrate on which BTO thin filmsare to be deposited by PLD. To decrease metal foil oxidation,we first deposited 6000 pulses at a vacuum of 0.01 mTorr,and then deposited the remaining numbers of pulses toachieve the desired thickness at an oxygen pressure of100 mTorr. From the SEM image of the cross-section (pleasesee Figure 3(a)), we can see that the BTO film was about700 nm in thickness. Unlike the epitaxially grown BTO filmson STO and Pt-buffered Si (which had grains that grew outperpendicular from the substrates), the BTO films on Ni foilwere polycrystalline with a small BTO grain size. In addition,the Ni surface was not as smooth as the STO substrate afterdeposition at a temperature of 750◦. Even though we haddeposited 6000 pulses in a vacuum, a thin layer of NiOstill formed at the interface (please see Figure 3(b)). Thehighest intensity peak for NiO at 43.3◦ was weaker than thehighest intensity one for BTO at 31.4◦. Please note that onlyNi, BTO and NiO phases were identified by the XRD linescan (Figure 3(b)). The NiO layer may serve as an importantbuffer layer, preventing further Ni oxidation.

The P-E loop, given in Figure 3(c), shows a saturationpolarization of Ps ≈ 6μC/cm2 and a remnant polarization ofPr ≈ 5μC/cm2. Both of these values are notably decreased,compared to BTO films grown on STO or Pt-Si. Therounding of the P-E loop indicates that there is some lossintroduced due to using Ni metal as the substrate andelectrode. From the M-H loop shown in Figure 3(d), we cansee that a saturation magnetization of Ms ≈ 47 emu/g wasachieved for this heterostructure. Compared with a pure Ni

1 μm

BTO

Pt

Si

Cross CFBTO on Si WD = 5 mm EHT = 5 kV Date: 8 Jul 2009

Mag = 10 KX Photo no. = 5482 Signal A = Inlens

(a)

Pt(111)

BTO(111)

Si(100)

7060504030

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−40

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Figure 2: Cross-section SEM (a), XRD (b), and polarizationmeasurement of BTO thin film grown on a Pt-buffered Si film.

foil which has Ms > 50 emu/g, we can see that Ms decreasedslightly because of the formation of a parasitic thin NiO layer,but yet still retained a good magnetization.

4 Journal of Nanomaterials

1 μm

BTO

Ni substrate

(a)

NiNiOBTO

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Ni foilNi foil with BTO

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80M

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Figure 3: (a) Cross-section view SEM images of BaTiO3 film on Ni foil, (b) is the XRD line scan, (c) is P-E loop from this film, and (d) H-Mloop measured from a pure Ni foil and Ni foil after depositing BTO layer.

After comparing the ferroelectric polarization of BTOthin films on different substrates, we could see that BTOon STO single-crystal shows highest polarization value,followed by BTO on platinized silicon and BTO on Ni foilshows the lowest polarization value. BTO on STO substrateshows obvious epitaxial growth property and this structureguarantees well-crystallized uniform BTO thin films whichfavors long ordering of the electric dipoles of BTO. BTOgrown on platinized Si is highly textured structure with only(111) XRD peak while BTO on Ni shows obvious polycrystalXRD peaks with a rough interface, thus BTO on Pt/Si showsmuch larger polarization property than BTO on Ni foil.

4. Summary

We successfully deposited ferroelectric BTO thin films onSTO, Pt-buffered Si and Ni substrates. We achieved epitaxialgrowth of BTO on Pt-buffered silicon substrates for the firsttime, which had much better ferroelectric properties of Ps ≈27μC/cm2 than previous polycrystalline BTO films on Pt-buffered Si. We also achieved deposition of BTO films on Nifoils for the first time. These Ni-BTO heterostructures werepolycrystalline, had a thin parasitic NiO layer that reducedPs and Pr , and slightly suppressed Ms. However, they offerpromise with additional research of being multiferroic.

Journal of Nanomaterials 5

Acknowledgments

The authors gratefully acknowledge the financial supportfrom National Science Foundation under contract no. DMR-0757502, and the Air Force Office of Scientific Research. Andthey also thank NCFL in VT for SEM, EDX work, and otheruseful discussions.

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[13] I.-D. Kim, Y. Avrahami, H. L. Tuller, Y.-B. Park, M. J. Dicken,and H. A. Atwater, “Study of orientation effect on nanoscalepolarization in BaTiO3 thin films using piezoresponse forcemicroscopy,” Applied Physics Letters, vol. 86, no. 19, Article ID192907, 3 pages, 2005.

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